bootstage: Support SPL

At present bootstage only supports U-Boot proper. But SPL can also consume
boot time so it is useful to have the record start there.

Add bootstage support to SPL. Also support stashing the timing information
when SPL finishes so that it can be picked up and reported by U-Boot
proper. This provides a full boot time record, excluding only the time
taken by the boot ROM.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/Kconfig b/common/Kconfig
index a3094d9..c49199b 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -18,6 +18,15 @@
 	  Calls to show_boot_progress() will also result in log entries but
 	  these will not have names.
 
+config SPL_BOOTSTAGE
+	bool "Boot timing and reported in SPL"
+	depends on BOOTSTAGE
+	help
+	  Enable recording of boot time in SPL. To make this visible to U-Boot
+	  proper, enable BOOTSTAGE_STASH as well. This will stash the timing
+	  information when SPL finishes and load it when U-Boot proper starts
+	  up.
+
 config BOOTSTAGE_REPORT
 	bool "Display a detailed boot timing report before booting the OS"
 	depends on BOOTSTAGE